1. Field of the Invention
The present invention relates to a method for producing a bumper reinforcement curved in an extending direction.
2. Description of the Related Art
A bumper reinforcement is made of a curved bumper reinforcement having a hollow, substantially rectangular cross section composed of a front face, an upper face, a lower face and a back face. The bumper reinforcement is provided at first as a straight tubular member having a substantially rectangular cross sectional shape constructing a front face, an upper face, a lower face, and a back face. Continuously, this straight tubular member is curved into a curved bumper reinforcement in accordance with the design of a vehicle by performing the front face of the straight tubular member as an outer periphery of the curved bumper reinforcement and by performing the back face of the straight tubular member as an inner periphery of the curved bumper reinforcement. The curved bumper reinforcement obtained is finally cut and separated at a predetermined production length to provide the bumper reinforcement.
This bumper reinforcement absorbs an impact applied to the vehicle bumper utilizing by a plastic deformation of aforementioned faces of the straight tubular member. However, occurrence of partial buckling on those faces while deforming by the impact may cause a serious deterioration of the load absorbing function of the bumper reinforcement for securing the vehicle""s safety. Therefore, the individual faces are required to have rigidities in the extending direction to restrain the partial buckling so that they may plastically deform not partially but over a wide range. The rigidities are obtained, as disclosed in Japanese Patent No. 3204635, by constructing a reinforcing rib between the front face and the back face of the bumper reinforcement.
The straight tubular member is relatively stretched on its front face and compressed on its back face when it is curved for fitting the shape corresponding to the design of the vehicle. The difference of the factor in the stretch and compression between the front face and the back face causes to generate wrinkles mainly on the angle portions formed between the upper face and the back face and between the lower face and the back face. The wrinkles are generated easier by the longer distance between the front face and the back face, i.e., the longitudinal width of the bumper reinforcement, by the smaller radius of curvature Ra of the bumper reinforcement, by the less ductility the plate member constructing the bumper reinforcement, and by the thinner plate member constructing the bumper reinforcement. Additionally, the wrinkles particularly formed on the angle portions between the upper face and the back face and between the lower face and the back face may cause to obstruct for fitting a bumper supporting member to be mounted on the bumper reinforcement.
The bumper reinforcement having the radius of curvature Ra is produced by curving the straight tubular member gradually, while being loaded several times in the curving direction, on the portion which is sandwiched between an inner die, as applied from the inside of the straight tubular member with profiling the cross sectional inside shape of the product, and an outer die, as applied from the outside of the straight tubular member as well as the inner die. Thus, it is conceivable to prevent occurrence of the wrinkles if the inner die and the outer die are brought in close contact with the individual faces of the straight tubular member.
In this method, however, the inner die and the outer die contact only partially with the straight tubular member so that the wrinkles are still generated at the portions which are out of contact with the inner die and the outer die while being loaded. It is also conceivable to restrain occurrence of the wrinkles controlling by that an amount of curvature applied to the straight tubular member at one time is reduced but the number of loading times to the member is increased. In this method, however, the high productivity can not be achieved as a result of deterioration of the production speed. If the number of the loading times applied in constant, on the other hand, the difference in the radius of curvature Ra causes to generate the wrinkles. This means that it is necessary to provide a plurality of additional production facilities for applying a different number of the loading times, in every different radius of curvature Ra required. Therefore, the inventors of the present invention have discussed to provide a production method which can prevent a bumper reinforcement from generation of the wrinkle without deteriorating the production speed, and, which can produce bumper reinforcements having different radius of curvatures Ra in common production facilities.
The bumper reinforcement in the present invention is produced in the following procedure. Specifically, the following procedure shows a method for producing a bumper reinforcement continuously from a curved bumper reinforcement having a hollow, substantially rectangular cross section composed of a front face, an upper face, a lower face and a back face. The production method in the present invention comprises the step of providing a straight tubular member by folding a plate member to construct a front face, an upper face, a lower face and a back face; the step of forming a curved bumper reinforcement, having the front face as an outer periphery of the curvature and the back face as an inner periphery of the curvature, obtained continuously by curving the straight tubular member while being sandwiched between an inner die and an outer die; and the step of cutting the formed curved bumper reinforcement to a predetermined length of the product. Further, in the present invention, corner portions of the inner die located on the side of a back face of the straight tubular member have radius established in larger than the radius of corner portions of the outer die located corresponding individually to the corner portions of the inner die, thereby forming the relief spaces M individually between the inner die and the outer die. Furthermore, the angle portions of the straight tubular member formed between the upper face and the back face and between the lower face and the back face have the radius profiling to the corner portions of the inner die. The method for producing a bumper reinforcement in the present invention is also characterized in that the relief spaces M in the present invention allow bulging of the angle portions of the straight tubular member in a range of the space itself, and thereby prevents generation of wrinkles particularly at the angle portions of the straight tubular member while being curved.
In this production method, the angle portions, as rounded in advance, of the straight tubular member are bulged in the curving procedure thereby to absorb the excess portions of the plate member, as might otherwise cause generation of the wrinkles. Accordingly, the method of the present invention can be applied to the straight tubular member which is formed by an extrusion molding of the light metal (e.g., aluminum or aluminum alloy), or to the straight tubular member which is integrally formed by folding a metal plate member. Moreover, the straight tubular member may have an opened cross sectional structure with an opened back face or a closed cross sectional structure with a closed back face. In the straight tubular member having the opened cross sectional structure, a pair of a rib face folded in or back from the upper face and the lower face is deemed as the back face thereof.
Here, it is advisable that the radius of the individual corner portions located on the side of the back face of the inner die is established in a range of three times to ten times as large as the thickness of the plate member. It is preferable that the radius of the individual angle portions of the inner die is established in a range of four times to six times as large as the thickness of the plate member. In this case, no wrinkle is generated if a range of the radius of curvature is established from 1,000 mm to 10,000 mm. That is, the radius of the angle portions of the straight tubular member is established in a range of about three times to ten times as large as the thickness of the plate member by profiling the corresponding corner portions of the inner die. On the other hand, the radius of the angle portions of the curved bumper reinforcement is to be less than ten times as large as the thickness of the plate member by profiling the corresponding corner portions on the side of the back face of the outer die. If the radius of the individual corner portions on the side of the back face of the inner die is less than three times as large as the thickness of the plate member, a sufficient amount of bulging of the angle portions cannot be secured. This means that it might be caused generation of the wrinkles by the individual corner portions of the outer die allowing a small amount of bulging of the angle portions of the straight tubular member while being curved. If the radius of the individual corner portions of the inner die is more than ten times as large as the thickness of the plate member, on the other hand, the angle portions of the curved bumper reinforcement may be excessively rounded to reduce the flat portion for mounting the bumper supporting member.
For absorbing the excess portions of the plate member causing generation of the wrinkles, it is preferable that the straight tubular member has the larger number of the angle portions rounded in advance. Accordingly, it is advisable that the back face of the straight tubular member having a flat configuration is reconstructed by arranging a concave groove therein into a back face having an irregularity configuration constructing an upper back face, a concave groove, and a lower back face. Additionally, it is also advisable that each of radius established to the corner portions of the inner die is profiled to apply for establishing the radius of the angle portions of the straight tubular member formed at boundary portions between the upper face and the upper back face, between the upper back face and the side surface of the concave groove, between the another side surface of the concave groove and the lower back face, and between the lower back face and the lower face. In other words, the concave groove formed in the back face increases the number of angle portions to absorb the excess portions of the plate member generating the wrinkles. For conveniences, the angle portions as the individual boundary portions formed between the upper face and the upper back face, and between the lower face and the lower back face will be called the xe2x80x9couter angle portionxe2x80x9d in the specification. Additionally, the angle portions as the individual boundary portions formed between the upper back face and the side surface of the concave groove, and between the lower back face and the other side surface of the concave groove will be called the xe2x80x9cinner angle portionxe2x80x9d in the specification.
The concave groove is provided to form the inner angle portions and is advantageous for enhancing the rigidity of the back face structurally. In the straight tubular member having the concave groove, it is also advisable that a reinforcing rib extending from the bottom surface of the concave groove to the front face is formed. At the curving treatment, the inner angle portions absorb the distortion generated by the reinforcement rib obstructing the compression of the back face, and thereby restrain generation of the wrinkles. Moreover, this reinforcing rib being shorter than the upper face and the lower face makes the bumper reinforcement advantageous for strengthening against the buckling.
In the straight tubular member further having a closed cross sectional structure formed by folding the plate member, it may be formed the straight tubular member by; one side edge of the plate member is folded in from the bottom surface of the concave groove to the front face through the angle portion and jointed to the front face to construct the reinforcing rib from the bottom surface of the concave groove to the front face, and the other side edge of the plate member is jointed to the angle portion or the reinforcing rib.
A welded mark jointing the two side edges of the plate member exists on the back face of the straight tubular member having a closed cross sectional structure formed by folding the plate member. These two side edges are welded on the bottom surface of the concave groove so that protrusion of the welded mark from the line linked between the upper back face and the lower back face is avoided. At this time, the two side edges to abut against each other are individually folded in toward the front face and jointed in contact-surface, and the valley formed by folding the two side edges is welded. Moreover, the valley formed by folding the two side edges provides a straight line formed by the folding treatment (especially by a roll-forming treatment). Therefore, it makes possible to use a laser welding treatment with a high welding speed. As a result, it brings speed up of the erosion rate to improve the productivity.
Moreover, in the case that one of the side edges is folded in toward the front face to construct the reinforcing rib, a laser welding treatment with a high welding speed can be utilized in welding the valley formed by folding those two side edges.